This invention relates to the construction of an aircraft so as to eliminate an undesirable flight characteristic exhibited by certain aircraft in which a nose-down pitching moment occurs as a function of increasing speed.
On present day swept wing jet transports, the axis of pitch of the airplane extends laterally through the center of gravity of the airplane and is typically located at approximately 25% of the mean aerodynamic chord of the wing. The center of lift of the wings are generally located rearward of the pitch axis. Thus, the lifting forces generated by the wings create a pitching moment that tends to force the nose of the airplane down. In stable horizontal flight, the pitching moment created by the lift of the wings is counteracted by a downward force exerted by the airplane's horizontal stabilizer.
On many airplanes, when increasing speed at a constant altitude, a nose-down pitching moment of increasing magnitude develops. This pitching moment is caused by the rearward shift of the center of lift of the wings as the airplane speed increases. As the nose of the airplane is pushed downwardly, the airplane picks up more speed, which in turn causes the pitching moment to increase, thereby causing a further increase in speed. The situation becomes progressively worse unless timely action is taken by the pilot or by some other means such as autopiloting controls.
This flight characteristic adds instability and may become more so due to an unpredictable flight environment, including local ambient wind patterns, e.g., gust upsets, wind shear and other disturbances.
The added flight instability places undesirable demands on the skill and alertness of the pilot who must compensate for the nose-down tendency, and for this reason, prior solutions to the problem have favored automatic controls, not dependent on pilot response.
One prior art solution has been the addition of an autopilot subsystem that senses the nose-down pitching moment and applies corrective elevator changes automatically. This approach works in counteracting the nose-down pitching moment; however, there are drawbacks. One is that the complex electrical and mechanical subsystems that make up the automatic control adds an element of unreliability. Other drawbacks include increased cost, weight and maintenance requirements. Moreover, on an airplane with a serious nose-down pitching moment problem, the authority demanded by an automatic system would be so high that it would introduce its own instability and other complexities, because additional subsystems would be needed to protect the airplane from hard over control actions.
It can be seen that when correcting the nose-down pitching moment of an airplane, it is desirable to have a mechanically simple system that is highly reliable and that will not add weight to the airplane or increase its cost or maintenance requirements.